Do lead bullets get harder with time?

That all depends on your alloy really. With lead bullets it all has to do with sizing the bullet to the correct diameter for best accuracy. I generally will size .002-.003 over bore diameter. So a .308 would be sized .310-.311, this allows for a good tight seal to help keep those hot gases behind the bullet. Which in turn helps keep the leading down and fouling up the barrel. Use a gas check and a good high velocity lube and you should find good success. I have a 300 WM Beragra barrel on a T/C Encore that with my load I hit 1650 FPS with a 195 grain bullet & 22 grains of SR4759 with unbelievable accuracy out to 200 yards. I use the rifle in a Schuetzen league and do pretty well with it. The load actually shoots better than I can.

I have another barrel on order, a 30-30 AI from MGM that I will be working up loads for as soon as it comes in. The smaller case of the 30-30 should give even better performance than the 300WM...time will tell!